Definition and properties of Lagrangian coherent structures from finite-time Lyapunov exponents in two-dimensional aperiodic flows

The standard SeaWiFS atmospheric correction algorithm, designed for open ocean water, has been extended for use over turbid coastal and inland waters. Failure of the standard algorithm over turbid waters can be attributed to invalid assumptions of zero water-leaving radiance for the near-infrared bands at 765 and 865 nm. In the present study these assumptions are replaced by the assumptions of spatial homogeneity of the 765:865-nm ratios for aerosol reflectance and for water-leaving reflectance. These two ratios are imposed as calibration parameters after inspection of the Rayleigh-corrected reflectance scatterplot. The performance of the new algorithm is demonstrated for imagery of Belgian coastal waters and yields physically realistic water-leaving radiance spectra. A preliminary comparison with in situ radiance spectra for the Dutch Lake Markermeer shows significant improvement over the standard atmospheric correction algorithm. An analysis is made of the sensitivity of results to the choice of calibration parameters, and perspectives for application of the method to other sensors are briefly discussed. © 2000 Optical Society of America OCIS codes: 010.1290, 010.4450, 120.0280.

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Atmospheric correction of SeaWiFS imagery for turbid coastal and inland waters

Recently published data taken in wind-wave tanks and on the ocean are shown to yield a growth rate, β, describing the transfer of energy and momentum directly from wind to surface waves, that is well described by the relation β = [(0.04±0.02)u*2ω cos θ]/c2 over a wide range of frequencies. Here u* is air friction velocity, ω is radian wave frequency, c is phase speed, and θ is the angle between wind and waves. Using this form and the requirement that the momentum flux from wind to waves not exceed the wind stress, we show that the total mean-square, upwind/downwind wave slope between the frequencies g/2πU10 and 20 Hz must be less than ρa/[ρω(0.04±0.02)], where ρa and ρω are air and water densities, g is gravitational acceleration, and U10 is wind speed at 10 m. Measurements of mean square slope both in wavetanks and on the ocean seem to show agreement with this limitation as long as the air-water interface is well defined. One implication of such a slope limitation is that mean slope spectral densities are limited to values which decrease as the peak frequency of a wind wave spectrum decreases. This may provide an explanation for the observation that mean-square, dominant wave slopes are typically smaller on the ocean than in wavetanks.

A relationship between wind stress and wave slope

A series of spatial wave images recorded by a conventional marine radar is analyzed to determine the three-dimensional E(kx, ky, ω) spectrum. In the absence of a surface current the spectral energy in this three-dimensional wave number frequency space will lie on a shell defined by the dispersion relationship. Any deviation from the expected dispersion relationship can be interpreted as being due to a current induced Doppler shift of the wave frequency. A least squares curve fitting technique is used to determine the surface current required to account for the observed Doppler shift. A comparison of the radar determined spectra and surface currents with ground truth data indicates that the radar system and analysis technique produces results consistent with conventional instrumentation.

/pdf/a-three-dimensional-analysis-of-marine-radar-images-for-the-hsgarp0bz2.pdf

A three-dimensional analysis of marine radar images for the determination of ocean wave directionality and surface currents

The median Doppler shift of radar echoes is analyzed in measurements by ENVISAT's Advanced Synthetic Aperture Radar (ASAR) over the ocean. This Doppler centroid differs from a predicted signal based on the predicted motion of the satellite and Earth. This anomaly, converted to a surface Doppler velocity U D , appears to be of geophysical origin. Two wide-swath images over the Gulf Stream around Cape Hatteras suggest that U D contains high-resolution information on surface currents, while on a global scale, U D is found to vary with the wind speed in the range direction. A simple quantitative forward model is proposed, based on a practical two-scale decomposition of the surface geometry and kinematics. The model represents the effect of the wind through the wave spectrum, and gives U D ≈ γU 10 ∥ + U c ∥ , with U 10 ∥ and U c ∥ as the 10 m wind speed and quasi-Eulerian current in the line of sight of the radar projected on the sea surface, respectively, and γ as a coefficient function of the wind speed, wave development, and radar geometry. It is found that for an incidence angle of 23°, γ ≈ 0.3 for moderate winds and fully developed seas. This model is validated with a global data set of ASAR Wave Mode observations, with colocated model winds, acquired over the global ocean during the years 2003 and 2004. The Doppler signal therefore provides the signed parameter U D that can be used to reduce the wind direction ambiguity in the inversion of high-resolution wind fields from SAR imagery. A qualitative validation of current effects is shown for the English Channel where tidal currents dominate. Thus it should be possible to combine this previously ignored geophysical Doppler signal with traditional information on sea surface roughness, in order to provide very high resolution wind and current fields.

/pdf/direct-measurements-of-ocean-surface-velocity-from-space-3bb03mvyya.pdf

Direct measurements of ocean surface velocity from space: Interpretation and validation

HF radio measurements of surface currents

Immediately following the passage of polar fronts that produced strong winds blowing away from Galveston Island and into the Gulf of Mexico, we used scattered LORAN-A radio waves, together with observations by wave stags, to measure the growth of 80 m waves as a function of angle to the wind, and the decay of the same wavelengths going against the wind The LORAN signals at 2 MHz are in Bragg resonance with 80 m (7 s) ocean waves offshore, and were used to map the strength of both approaching and receding 80 m waves as a function of range and azimuth using Doppler velocity to determine wave directions We found that 1) these waves grow as the cosine of the angle to the wind; 2) growth rates averaged over all angles are comparable to those previously published, 3) growth rates measured as a function of time by the wave staff were numerically the same as growth measured as a function of distance by the radar; 4) growth appears to be inhibited for waves having frequencies near the peak frequency of 

Dekameter Radar Observations of Ocean Wave Growth and Decay

The Coastal Zone Color Scanner on the Nimbus-7 satellite produces images of the sea in five colors (bands) of light, and these can be used to measure chlorophyll concentrations in the water, to visualize chlorophyll patterns, and to observe how these patterns change through time. However, 80-90% of the light observed by the instrument comes from the atmosphere, and removing this air light is difficult and usually requires considerable processing of the data. We describe here a new method to remove the influence of air light that is accurate, independent of external information, and requires far fewer computations than previous methods. This procedure should facilitate the use of CZCS data in biological oceanography, especially by investigators who may not have access to large computers and elaborate atmospheric correction procedures. The technique has been tested by comparing ship and satellite estimates of chlorophyll; the comparison shows very good agreement. In addition, a comparison of sequential chlorophyll images shows that the technique produces stable chlorophyll estimates under changing atmospheric conditions.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1985.30.2.0273

The atmospheric correction and measurement of chlorophyll concentration using the coastal zone color scanner1

HF radio waves backscattered from the ocean surface can be used to measure ocean surface currents. Measurements of the range-Doppler spectrum of these signals yields the wavelength and frequency of the scattering ocean waves, and thus their phase velocity. This velocity is subtracted from the velocity the wave would have in still water, and the difference is a measure of the average (over depth) of the surface current. To test these ideas, we measured currents using a conventional and the IiF technique, and found reasonable agreement between the two measurements.

Robert H. Stewart

Papers

HF radio measurements of surface currents

Dekameter Radar Observations of Ocean Wave Growth and Decay

The atmospheric correction and measurement of chlorophyll concentration using the coastal zone color scanner1

Hf radio measurements of ocean currents